LAUSR

ABSTRACT In the growing degree days (GDD) model for plants, 0°C or 5°C have been empirically used as base temperature (BT) for the GDD model, without experimental determination. In this study, developmental periods for bud break and leaf opening of mizunara oak, Quercus crispula were determined by using samples from locations separated by 900 km, cut branches from central Japan and potted saplings from northern Japan. The samples from each location were then grown in growth chambers under the four constant temperatures in different years. The relationships between the developmental rate and temperature were determined, BTs were calculated by regression analyses, and the required GDDs were determined. Multiple regression was applied to compare model fittings for bud break and leaf opening. Observational data were fitted with a logistic curve to empirically determine the BT and GDD for bud break. Better fitting was obtained for bud break than for leaf opening, in the growth chamber experiment as well as in the multiple regression, probably because bud break was controlled by the GDD accumulation, while the rate of leaf opening was directly and exponentially influenced by temperature. The BT for bud break obtained by the growth chamber experiments varied from 0.09°C–4.88°C between the two locations, and also within one location by year. A large variation in the GDD accumulation was recognized in the field observation at each location. These results indicate additional environmental factors not included in the GDD calculation likely influenced the leafing phenology of Q. crispula.